The invention relates to a method and an arrangement for controlling the position and/or force of an elongated rolling device in the roll nip between two elongated rolling devices in paper and board machines.
The nip pressure in a roll nip between two rolls and the opening and closing of the roll nip are adjusted with hydraulic means connected to said rolls, such as hydraulic cylinders. For nip pressure control, suitable measuring means are first used for measuring the force/pressure generated by the roll in the roll nip, the control logic of the control system converts an analog measurement signal into a digital signal and transmits a control signal in digital form to the control valve in charge of changing the nip pressure. The digital control signal is converted into analog form by the control valve, and then the control valve controls the fluid flow entering and leaving the hydraulic means. Such a manner of controlling nip pressure has noticeable shortcomings, of which the major ones relate to disappearing data content as an analog measurement signal is converted into digital form and a digital control signal is subsequently converted into a control signal.
There are frequently also problems caused by the fact that the same relatively large-sized control valve, such as a proportional valve, is used for controlling both the force exerted by the roll on the backing roll in the roll nip between the rolls and also the roll position relative to the backing roll. This problem is particularly tangible in reelers, because, as the fiber web is reeled around the reel core, the reel core needs to be continuously shifted away from the reeling cylinder. However, meanwhile it is necessary to maintain the nip pressure between the reel core and the reel cylinder on a determined level. The shift of the location of the reel core requires relatively large movements of the piston of the hydraulic means and also changes of the fluid pressure prevailing in the compression cylinder, whereas changes of the nip pressure can be achieved with considerably smaller piston movements and changes of the fluid pressure in the compression cylinder, entraining a tendency to cause control fluctuation and vibrations in the roll/rolls. In practice, due to the great mass of the control valve and the consequently slow changes of the flow volume in the hydraulic means, it is often difficult or even impossible to actively attenuate roll vibrations caused by control fluctuation by means of control engineering means.
Controlling hydraulic means by current control valves such as servo valves and proportional valves is awkward and inaccurate, because the required valves are bulky and slow, and thus have poor control resolution. In addition, the control valves themselves might cause control fluctuation and vibrations in the rolling devices by their own operation.
The purpose of the invention is to eliminate the prior art inconveniences. Thus, the first purpose of the invention is to achieve a system for controlling the location and the force of the roll, allowing the same hydraulic means to accurately control both the location of the roll relative to the backing roll in the roll nip and also the nip pressure (=force) generated by the roll in the roll nip, substantially without control fluctuation. A second purpose of the invention is to achieve an active manner of control enabling efficient attenuation of roll vibrations.